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Multidentate Pyridyl-Based Ligands in the Coordination-Driven Self-Assembly of Palladium Metallo-Macrocycles

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Abstract

In the current study, a convenient way is presented to synthesize one ditopic and two tetratopic pyridine-based ligands, which are then used to construct metallo-supramolecular polygons. The tetratopic ligands offer two different metal-binding sites, one central 2,2′-bipyridine, which can act as a chelate ligand, and two separate pyridine rings, which mediate assembly formation. The three ligands differ with respect to their conformational flexibility. While a biphenyl core allows the ligand to adjust its conformation as needed, a bipyridine core strongly prefers a divergent arrangement of the additional pyridine binding sites, but can be fixed in a cisoid conformation by metal complexation to the bipyridine.Instead, a phenanthroline core already fixes the pyridinylethynyl substituents in a cisoid structure without any metal coordinated to it. Upon mixing each one of the ligands separately with the appropriate amount of dpppPdII triflate, discrete self-assembled metallo-macrocycles are formed which are characterized by 1H and 31P NMR spectroscopy and mass spectrometry. Mixing all three ligands simultaneously with the metal complex leads to the formation of a statistical dynamic combinatorial library (DCL) of all possible homo- and heterodimeric metallo-supramolecular assemblies. This underlines the conformational differences between the three ligands not to impact significantly on the self-assembly process.

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